It's because of how ui_handle_menu_event works -- it makes quite tricky checks in which direction movement happens depending on button type.

Not sure what's easier/safer: change logic of this function or change type of buttons used for nodes menu. Checked usages for uiItemV and found that it's used in node_editor only, so changes type of button used there. Attaching patch and assigning to Brecht to check if it's acceptable change.